Demands for high-power power-frequency uninterruptible power supply (UPS) are very large in global industrialization and informatization economic construction. In the field of medium-high-power UPS, the power-frequency UPS has the advantages of good adaptive capacity to power supply environment, high reliability and strong shock resistance of load capacity and the like, thus being widely used in all industries and fields. Particularly, independent and stable zero potential unique to the power-frequency UPS provides a guarantee for safety use of the high-power UPS. Grid-side current of a conventional power-frequency UPS has large current harmonic, which causes serious pollution to power grid; at present, high-power UPS is configured with a power grid compensation device and the like so as to meet the requirements for use of power grid.
In order to solve the problem of power harmonic pollution resulted from input side rectification of the conventional high-power power-frequency UPS, a full-isolation phase-shifting transformer based on “Δ/Y-type and Δ/Δ-type structure” and a multi-pulse rectifier structure of a balance reactor are adopted for input of a high-power system. For example, 12-pulse rectification technology is achieved by two conventional 6-pulse phase-controlled rectifiers connected in parallel with a phase-shift angle of 30° so as to suppress quintuple or septuple harmonic produced by a three-phase six-pulse rectifier. The method has the problem of high cost and the full-isolation phase-shifting transformer is large and heavy, inconvenient for extending of power unit of a higher-power AC-DC rectifier. 18-pulse rectification technology can further reduce grid-side current harmonic, a nine-phase phase-shift power supply thereof is realized usually by an isolation transformer, which has disadvantages of bulky isolation transformer and low economic benefit.